Fabrication method for arranging ultra-fine particles

ABSTRACT

A method and resultant device, in which metal nanoparticles are self-assembled into two-dimensional lattices. A periodic hole pattern (wells) is fabricated on a photoresist substrate, the wells having an aspect ratio of less than 0.37. The nanoparticles are synthesized within inverse micelles of a polymer, preferably a block copolymer, and are self-assembled onto the photoresist nanopatterns. The nanoparticles are selectively positioned in the holes due to the capillary forces related to the pattern geometry, with a controllable number of particles per lattice point.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional of application Ser. No. 10/759,589filed Jan. 15, 2004 now U.S. Pat. No. 6,989,324.

FIELD OF THE INVENTION

The field of art to which the invention pertains relates to thefabrication methods that enable control of the number and positioning ofa fixed number of ultra-fine particles or nanoclusters arranged intowells prefabricated on substrates.

BACKGROUND OF THE INVENTION

Micro- or nano-patterns of nanoparticles have recently attractedconsiderable interest due to their possible applications in high-densitymagnetic storage media (S. Sun et al. 2000), magneto-optical devices (H.Akinaga et al. 2000), and quantum dot displays (K. Suzuki et al. 1999).To obtain these particle patterns, various methods have beensuccessfully applied such as selective wetting on a patternedself-assembled monolayer (SAM) (S. Palacin et al. 1996), layer-by-layer(LbL) self-assembly on a patterned SAM (H. Zheng et al. 2002; and I. Leeet al. 2002), LbL self-assembly combined with lift-off and/or metal masktechniques (F. Hua et al. 2002a; F. Hua et al. 2002b), electron beamlithography after deposition of a nanoparticle layer (X. M. Lin et al.2001; M. H. V. Werts et al. 2002), and micro-contact printing (μCP) ofnanoparticles using soft poly(dimethylsiloxane) (PDMS) network stamps(P. C. Hidber et al. 1996 and Q. Guo et al. 2003). However, the first(S. Palacin et al. 1996) and the second methods (H. Zheng et al. 2002;and I. Lee et al. 2002) need pre-patterned SAM layers on a substratetypically prepared by μCP with PDMS stamps, which involves additionalfabrication steps of original masks and stamps. The third method (F. Huaet al. 2002a; F. Hua et al. 2002b) involves many steps such asconventional lithography to prepare photoresist patterns and lift-offand/or metal mask after LbL self-assembly. In the fourth technique (X.M. Lin et al. 2001; M. H. V. Werts et al. 2002) it is crucial to preparedense mono- or multilayers of nanoparticles on a substrate surface,which may not be easy in a large area fabrication. The last method (P.C. Hidber et al. 1996 and Q. Guo et al. 2003) also needs dense mono- ormultilayers of nanoparticles on a PDMS stamp and sometimes additionaladhesion promoters are needed for smooth pattern transfer to anothersubstrate. With the exception of e-beam lithography (M. H. V. Werts etal. 2002) the size and periodicity of patterns obtained by the abovemethods are typically on the order of microns.

Recently, a few research groups used the capillary forces of a recedingliquid front to self-assemble particles into physically templated wellson both micrometer and nanometer scales (Y. Yin et al. 2001, J. P. Spatzet al. 2002, and M. J. Misner et al. 2003). Particularly, Spatz et al.(2002) reported an ordering of singlepolystyrene-block-poly(2-vinylpyridine) (PS-PVP) micelle loaded withtetrachloroauric acid into each regularly-spaced hole of photoresistpatterns with an aspect ratio of a=0.4˜2.7 prepared by e-beamlithography. They observed a circular depletion zone withoutnanoparticles around a hole, which was consistent with the capillaryforce effect. Misner et al. (2003) also reported the self-assembly ofnanoparticles using capillary forces into block copolymer templates ofperpendicularly oriented cylindrical wells obtained by UV irradiation.They found that nanoparticles with a diameter larger than 10 nm couldnot be accommodated perfectly in the cylindrical wells due to the smalldiameter (˜20 nm) of the wells.

BRIEF SUMMARY OF THE INVENTION

The present invention overcomes the foregoing drawbacks by providing anovel method and resultant device, in which metal nanoparticles areself-assembled into two-dimensional lattices. A periodic hole pattern(wells) is fabricated on a photoresist substrate, the wells having anaspect ratio of less than 0.37. The nanoparticles are synthesized withininverse micelles of polymers, preferably block copolymers, and areself-assembled onto the photoresist nanopatterns. The nanoparticles areselectively positioned in the holes due to the capillary forces relatedto the pattern geometry, with a controllable number of particles perlattice point.

In a specific embodiment, monodisperse cobalt nanoparticles aresynthesized within inverse micelles of PS-PVP copolymer in toluene. Aperiodic hole pattern of photoresist is fabricated on a GaAs substrateby holographic lithography, and the nanoparticles as prepared above areself-assembled onto the photoresist nanopatterns by dip or spin casting.

Features and advantages of the invention will be described hereinafterwhich form the subject of the invention. It should be appreciated bythose skilled in the art that the conception and specific embodimentdisclosed may be readily utilized as a basis for modifying or designingother surfaces and substrates for carrying out the same purposes of thepresent invention. It should also be realized by those skilled in theart that such equivalent constructions do not depart from the spirit andscope of the invention. The novel features which are believed to becharacteristic of the invention, both as to its organization and methodof operation, together with further objects and advantages will bebetter understood from the following description when considered inconnection with the accompanying figures. It is to be expresslyunderstood, however, that each of the figures is provided for thepurpose of illustration and description only and is not intended as adefinition of the limits of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1( a) and 1(b) are respectively scanning electron and scanningforce microscope images showing height images of a 2-D nanowell patternof photoresist prepared by holographic lithography;

FIG. 2 is a scanning electron microscope image of cobalt nanoparticlearrays obtained by dip-coating at 100 μm/s onto a photoresist holelattice, along with an inset showing a higher magnification;

FIGS. 3( a), 3(b) and 3(c) are scanning force microscope height imagesof cobalt particle arrays using nanoparticle concentrations respectivelyof 0.125 mg/ml (FIG. 3( a)), 0.5 mg/ml (FIG. 3( b)), and 1.0 mg/ml (FIG.3( c)); and

FIG. 4 is a graph showing the relationship between nanoparticle numberper hole and nanoparticle concentration.

DETAILED DESCRIPTION OF THE INVENTION

It is to be understood that unless otherwise indicated, this inventionis not limited to specific materials, processing conditions,manufacturing equipment or the like, as such may vary. It is alsounderstood that the terminology used herein is for the purpose ofdescribing particular embodiments only and is not intended to belimiting.

Definitions

The term “aspect ratio” is used herein in its conventional sense torefer to the ratio of the height or depth to width (or diameter) of anobject.

The term “nanoparticle” is used herein in its conventional sense torefer to ultra-fine particles (below 50 nm).

The Process

The invention relates to a fabrication method that enables control ofthe number of ultra-fine particles that are arranged into wellsprefabricated on substrates.

The nature of the substrate is not critical for the process although itmay affect the method of deposition of the photoresist and the solventused for the deposition. The most commonly used substrate has beensilicon wafers. Depending upon the application, these wafers can becoated with other layers such as dielectric layers, photoresist orpolyimide, metal oxides, thermal oxides, conduction materials,insulating materials, ferroelectric materials or other materials used inthe construction of electronic devices. Materials for substrates otherthan silicon include, but are not limited to, GaAs, InGaAsP, InGaP, orany other alloys of Ga.

The substrate is then coated with a layer of a suitable photoresist witha suitable thickness. Photoresists are organic polymers which becomesoluble when exposed to ultraviolet light. Any number of photoresistscan be used depending on how thick a layer of photoresist is required.Photoresists are essentially positive or negative type. Negativephotoresists are a type of photoresist that becomes relatively insolubleto developers when exposed to light. In contrast, positive photoresistshave a higher developer dissolution rate after being exposed to light.Positives are more commonly used because they do not swell duringdevelopment; they are capable of finer resolution, and they arereasonably resistant to plasma processing operations. Examples ofpositive photoresists include, but are not limited to, Shipley 3001,3612 and 220-7. The photoresist is coated onto the substrate using anynumber of conventional techniques such as spin-coating, dipping or othercoating methods.

The photoresist coated-substrate is then typically but not necessarilyprepared using optical, X-ray, electron beam, LIGA, or holographiclithography. Holographic lithography involves making patterns using abeam of laser light that interferes with itself reflected by a mirror.The dimensions of the pattern produced are determined by the number ofbeams used. Thus, a 2-D pattern of cylindrical holes and or 1-D patternof grooves can be produced. The final pattern achieved is determined byfactors such as the wavelength of the laser light, the relative anglesat which the beams are fired, and photoresist exposure time andthickness. Although other laser sources and wavelengths could have beenchosen, we used a He—Cd laser operating at 325.0 nm. Once the patternhas been fabricated, the substrate is ready to be coated with micelleswhich contain the nanoparticles of interest.

Nanoparticle composition is dependent upon the application. For example,rare earth magnetic materials composed of single domain particles couldbe patterned in this process and used in computer memory applications.Ferroelectric nanoparticle components could be used for capacitors.Cobalt particles were used here because of their useful magneticproperties. Other materials include but are not limited to FeO, Fe₂O₃,Fe₃O₄; FePt, MnAs, Ga—Sb, Au, Ag, Pt, Pd, and Ni. These nanoparticlescan be synthesized in inverse micelles of polystyrene block copolymersin a solution of toluene. Although the invention usedpolystyrene-block-poly(2-vinylpyridine) (PS-PVP), any number ofpolystyrene block copolymers could have been used. When dissolved intoluene, these molecules aggregate into micelles with a PS shell and aPVP core with a controllable hydrodynamic radius.

The micelle-nanocluster solution is then used to coat the prefabricatedsubstrate. Coating can be performed by casting, dipping, orspin-coating. For instance spin-coating speeds such as 1,000 rpm togreater than 10,000 rpm can be employed to control the thickness of thecoating. Nevertheless, whichever technique is used, it is possible tofine tune the number of nanoparticles that will finally coat the wellsby adjusting the nanoparticle concentration in the solution used in thecoating process. Nanoparticle number per well may vary depending uponthe nanoparticle/micelle composition. Nanoparticle concentrations ofless than 0.1 mg/ml to greater than 1 mg/mL can be used. Examples inthis application used Co nanoparticles concentrations of between 0.125mg/ml and 1 mg/mL.

The coating method results in micelles/nanoparticles being positionedwithin fabricated substrates through capillary forces that act at thecontact line between a micelle/nanoparticle solution and a wall.However, the prefabricated substrate may also contain micelle-freeregions. This results from a specific combination of the resist, thesolvent, the micelle-forming polymer with respect to their interactionsand surface energies.

The following examples set forth for purposes of illustration only andare not to be construed as limitations on the invention except as setforth in the appended claims.

EXAMPLE 1

A PS-PVP copolymer with a weight average molecular weight of 65,200g/mol, a polydispersity of 1.04, and a PVP volume fraction of 0.12 wassynthesized by sequential anionic polymerization technique as previouslydocumented. (H. Yokoyama et al. 2000). Co nanoparticles were synthesizedin inverse PS-PVP micelles in toluene by partial pyrolysis of dicobaltoctacarbonyl, Co₂(CO)₈, at 115° C. as reported elsewhere. (F. S. Dianaet al.). The synthetic scheme is a variation of the method described byPuntes et al. (2001). Monodisperse amorphous Co nanoparticles with anaverage diameter of 20±2 nm were obtained after a reaction time of 2min. The structure of micellar nanoparticles obtained is similar to thatof the PS-PVP micelles loaded with tetrachloroauric acid reported bySpatz et al. (2000).

Two-dimensional periodic nanopatterns were fabricated by means ofholographic lithography. The beam from a He—Cd laser operating atwavelength of 325.0 nm was expanded and spatially filtered using apinhole in order to select only the coherent central zone. This spot wasaligned towards the center of an interferometer with mirror and sampleplanes forming 90° dihedral angle. The body of the interferometer can berotated around the dihedral axis in order to adjust the periodicity ofthe grating patterns, while double exposures at different sampleorientations allow formation of grid patterns. (G. Gigli et al. 1998). A50 nm thick layer of a photo sensitive chemical resistant to acid,specifically Shipley SPR 3001, was used as a positive photoresist on aGaAs substrate. Precise calibration of the exposure dose made possiblethe definition of grid well nanopatterns with good process latitude overa large area (2×2 cm²).

A toluene solution of nanoparticles was then dip-or spun-cast atdifferent speeds onto these photoresist nanopatterns. Typically, adrawing rate of 100 μm/s was used for dip-casting and a spinning rate of2,000 rpm was used for spin-casting. The nanoparticle arrays obtainedwere characterized by scanning force microscopy (SFM) and scanningelectron microscopy (SEM). SFM was carried out using a DigitalInstruments Multimode Scanning Probe Microscope with a Nanoscope IIIacontroller in tapping mode. Field emission SEM was performed on JEOL6340F at an accelerating voltage of 2 kV.

FIGS. 1( a) and 1(b) show SEM and SFM height images, respectively, ofthe 2D square lattice of nanowells obtained by holographic lithography.In FIG. 1( a), bright spots correspond to wells, while dark spotscorrespond to the wells in FIG. 1( b). The periodicity of the wells was250 nm and the depth was 31 nm along line c and 39 nm along line d withabout 10 nm of photoresist left at the bottom of the wells. The slightdifference in the height of the photoresist pattern depending on thedirection comes from the fact that the photoresist is double exposed at90° to the beam with the same dose. This 2D pattern is consistent withthe calculated 2D intensity profile shown in the inset of FIG. 1( b).The well aspect ratio in our study is below 0.3, which is much smallerthan those in the works of Spatz et al. (2002) and Misner et al.'s(2003).

FIG. 2 shows an SEM image of the self-assembly of Co nanoparticlesobtained by dip-coating the nanoparticle solution (c=1.0 mg/ml) onto thephotoresist well patterns at 100 μm/s. The inset shows the same regionwith a larger magnification. The nanoparticles are selectivelypositioned in the wells. The higher magnification SEM image clearlyshows 6±2 particles per well on average. The selective positioning ofthe nanoparticles in our study is consistent with the capillary forceeffect. During dip- or spin-coating, the solvent-air interface recedesalong the photoresist surface with solvent in the nanowells evaporatinglast. The capillary force of the receding solvent-air interface acts onthe nanoparticles during this process and collects them in the wells.This observation is consistent with the results of Spatz et al. (2002)and Misner et al. (2003). However, it is surprising that the capillaryforces still work for the pattern with the very low aspect ratio (a˜0.3)of the wells in our study.

The PS shells of the Co nanoparticles as well as the solvent, toluene,are hydrophobic, while the photoresist surface is slightly hydrophilicdue to the hydroxyl (—OH) groups in the Novolac resin. This leads to therepulsive interaction between the photoresist surface and thenanoparticle droplets and further helps the assembly of thenanoparticles into the wells. Xia and coworkers (Y. Yin et al. 2001)reported poor self-assembly of —NH₂ terminated PS colloids at pH=6.5because the capillary forces are not strong enough to drive theparticles into the wells due to the strong attractive interactionbetween the colloids and the photoresist substrate. We observed that aphotoresist well lattice with no photoresist left in the wells (a thinnatural oxide layer of GaAs substrate exposed in the wells) gives nearlythe same result (not shown here) as those shown in FIG. 2. This impliesthat capillary force is more effective in the selective self-assemblyprocess in our system than the repulsive interaction between thenanoparticles and the photoresist.

In FIG. 2, only a few defects are observed in the area of 7.8×5.8 μm².The circle in FIG. 2 indicates a defect where two nanoparticle arrays inadjacent wells are connected by a chain aggregate of particles, whilethe rectangle shows another type of defect where nanoparticles aredistributed around the wells as well as in the wells. These kinds ofdefects originate from the nanoparticles aggregates present in thesolution rather than the pattern roughness or the possible flowinstability during dipping. The degree of aggregation in Co nanoparticlesolution, particularly chain aggregates, could be controlled by changingthe aging time at room temperature as well as the reaction time asdocumented elsewhere (F. S. Diana et al.).

EXAMPLE 2

The spin-coating method was also used to make the self-assemblednanoparticle arrays on the photoresist patterns, and the self-assembliesobtained at 2,000 rpm are shown in the SFM height images of FIG. 3(a)-(c). When a dilute solution (c=0.125 mg/ml) was used, arrays with asparse population of nanoparticles (2±2 particles per well) wereobtained without any particles positioned on top of photoresist mesaregions, as shown in FIG. 3( a). In FIG. 3( a), some wells are showncontaining only a single nanoparticle. A more concentrated solution(c=0.50 mg/ml) results in arrays with a dense population ofnanoparticles (9±2 particles per well), as shown in FIG. 3( b). Thenanoparticle number per well is linearly proportional to the solutionconcentration at low concentrations as shown in FIG. 4. This impliesthat the nanoparticle density in the wells can be controlled by simplychanging the nanoparticle concentration in the solutions used. If theconcentration is further increased to 1.0 mg/ml, however, the number ofnanoparticles per well deviates from the linear relationship and levelsoff to 14±2 as shown in FIG. 4. This is because each well canaccommodate only a certain number of nanoparticles due to the limitationof its size. Excess particles remain on top of mesa producing defectparticles as shown in FIG. 3( c).

The invention demonstrates that capillary forces of a receding liquidfront can be used to selectively self-assemble nanoparticles intophotoresist nanowells.

REFERENCES

The following publications are referred to in the above specification:

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1. A fabricated device comprising: a photoresist coated-substrate formedwith wells through its surface, said wells having gradually slopingwalls and an aspect ratio of less than 0.37, and micelles enclosingnanoparticles disposed in said wells.
 2. The fabricated device of claim1 in which said wells have an aspect ratio less than or equal to 0.3. 3.The fabricated device of claim 1 wherein the nanoparticles are metalnanoparticles.
 4. The fabricated device of claim 3 wherein the metal iscobalt.
 5. The fabricated device of claim 1 wherein said nanoparticleshave a diameter of 0.5 to 500 nm and said wells are less than 1 μm indiameter.
 6. The fabricated device of claim 1 wherein the diameter ofsaid micelles in dry condition is 0.01 to 1.0 times the diameter of saidwells.
 7. The fabricated device of claim 1 wherein the diameter of saidmicelles in solution is less than 1.5 times the diameter of said wells.8. The fabricated device of claim 1 wherein said wells are arranged inan ordered manner.
 9. The fabricated device of claim 1 wherein saidwells are arranged in a random manner.
 10. The fabricated device ofclaim 1, wherein said wells have a bottom surface comprising of saidphotoresist.
 11. The fabricated device of claim 1, wherein said wellshave a bottom surface where the photoresist has been removed.
 12. Afabricated device comprising: a GaAs substrate coated with a photoresistlayer of a photo sensitive chemical resistant to acid, said substrateand photoresist layer defining wells that have gradually sloping wallsand an aspect ratio of less than 0.37, and micelles within said wells,said micelles incorporating cobalt nanoparticles in the device.
 13. Thefabricated device of claim 12 wherein said wells are in the form ofperiodic elongate grooves.
 14. The fabricated device of claim 12 whereinsaid wells are 100-200 nm deep.